Glasses that melt at relatively low temperatures have long been used to join metal, glass and ceramic components by a fusion seal. Initially, such seals were vitreous in nature. Lead glasses, particularly lead zinc borates and borosilicates, were commonly used.
The sealing art was revolutionized by discovery that such glasses could be crystallized in a controlled manner by thermal treatment. This combined the virtues of a low melting glass with a strong seal in the crystalline state.
More recently, non-lead glasses, composed essentially of tin, zinc, and phosphorous oxides, have been proposed as a substitute for the traditional lead zinc borate glasses. U.S. Pat. No. 5,246,890 (Aitken et al.) discloses examples of such phosphate glasses.
In forming a seal with such sealing glasses, it has become customary to use the glass in a particle form known as glass frit. The glass frit is mixed with a vehicle and binder to form a suspension, or paste. This mixture is applied to a sealing surface, for example, by extrusion.
Numerous organic vehicles and binders have been proposed. A combination of nitrocellulose and amyl acetate is commonly used in commercial cathode ray tube production. Recently, environmental concerns about volatile organic compounds (VOCs) have given rise to a proposal for substitution of an aqueous solution of a cellulosic polymer.
In preparing a sealing glass paste, serious consideration must be given to the rheological characteristics of the paste, that is, its flow or viscosity characteristics. The paste needs to be relatively fluid for application purposes. However, both before and after application, when the paste is at rest, it should be relatively stiff or viscous.
Prior to application, there may be a substantial delay between preparation of the paste and its use. This may be due to transportation or storage. If the paste is not sufficiently viscous, settling or separation of components may occur.
Frequently, a paste is applied by being extruded as a ribbon of the form required for sealing. It is desirable that the paste set quickly after application so as to avoid any flow that would cause loss of the extruded shape.
In order to meet these contradictory requirements, it has been proposed to employ a paste having the characteristics of a reversible gel. Such a paste exhibits a high viscosity during storage and/or after application, but exhibits a much lower viscosity when placed under shear stress. This stress may be created in a mixer, or by pressure applied during extrusion.
U.S. Pat. No. 4,260,406 (Corbett et al.) describes introducing a gelling agent into a paste to impart the characteristics of a reversible gel. A number of organic and inorganic gelling agents are disclosed as being useful. However, certain surfaceactive organic titanates, known as titanate coupling agents, are indicated to be preferred.
This known technique of controlling rheology is effective. However, it would be desirable to provide even better control. It would also be desirable to obtain such control without resort to the known gelling agents. In particular, it would be desirable to employ only non-volatile, non-combustible, inorganic additives.
It is a basic purpose of the present invention to provide a method of rheology control in a sealing glass paste that achieves these ends. Another purpose is to provide a method of control that utilizes an additive having the desired characteristics. A further purpose is to provide a method that permits an extended delay between preparation of a paste and its use without settling, or other undesirable separation, occurring. A still further purpose is to provide a method of rheology control that permits relatively easy application of the paste to a sealing surface.